1. Field of the Invention
This invention relates to a method and apparatus for mixing gases such as a combustible gas and air, and further relates to a mixer capable of maintaining the gas-to-gas or air-to-gas ratio substantially constant even while the total of flow of the mixture considerably increases or decreases.
The invention is particularly beneficial as a mixing device in providing fuel burners with an advantageous "turndown" range, which is the range extending from maximum to minimum total fluid flow, through which range the mixing device is capable of maintaining the gas-to-gas or air-to-gas ratio substantially constant.
2. Prior Art
There are many needs for effective mixing of gases of various types. Examples include:
Mixing a fuel gas with air for combustion in a burner. PA1 Mixing gases such as hydrogen and carbon monoxide in order to provide a so-called carburizing medium. PA1 Mixing various gases such as propane and air in order to form a so-called blended gas to be used as a backup fuel for a system that normally uses natural gas.
In most instances there is a need not only to produce a mixture of different gases in predetermined ratios, but also to vary the total flow rate of the mixture without causing a significant change of the desired ratios.
Frequently, mixing devices are combined with fans, blowers, or compressors so that the mixture that is produced can be delivered at a controlled, elevated pressure. For combustion applications, the combination is called a mixing machine.
Many kinds of mixing devices have been commercialized. In all of them two or more fluid streams are brought together in some kind of device and leave as a single, mixed stream.
The most basic kind is called a mixing tee. FIG. 1 shows a conventional mixing tee as it would be applied to mixing fuel gas with air. For simplicity, the safety devices that normally would be present are not shown. A blower 12 takes in ambient air and raises its pressure in order to force it through the downstream elements of the system. An orifice 2 establishes a definite relationship between the flow rate of the air and a pressure drop across the orifice. Fuel gas is received from the mains, at a pressure greater than atmospheric, by a gas governor 10.
The gas governor reduces the pressure of the fuel gas, in a pipe 8 just upstream from an adjustable orifice 6, to a value equal to the air pressure measured just upstream from the air orifice 2. As the fuel and air pressures must be equal at the pipe tee 14 where the gas and air come together, the pressure differences across the two orifices must also be equal. Insuring that these two pressure differences are equal is the purpose of the gas governor. The composition of the air-fuel mixture, usually expressed as an air-fuel ratio, can be set to a predetermined value by adjusting orifice 6.
The conventional mixing tee has certain inherent problems that limit the range over which it can maintain a sufficiently constant mixture air-fuel ratio. These are:
1. The gas governor cannot set the inlet pressures of the two gases to be precisely equal. As the pressure differences for the air and the fuel gas become very low at low demand, the mixture composition fails to stay constant because the pressure drops of the gases become increasingly unequal with decreasing demand. This can be compensated by using a smaller air orifice. The pressure drop at minimum demand is then increased enough to make the effect of the gas governor error negligible. Replacing the air orifice with another of just the right size is a nuisance at best if field adjustments become necessary. More likely, there will be a serious delay while the correct orifice is being made.
2. The flow coefficient through an orifice or valve tends to have a constant value at high flow rates, or, more accurately, at high Reynolds numbers. (Reynolds number is a dimensionless quantity which, for the purpose of this invention, may be defined as the gas velocity multiplied by the gas density multiplied by the pipe diameter, just upstream of the valve or orifice, and divided by the gas viscosity.) Conversely, at low Reynolds numbers, the flow coefficient will vary rapidly with changes in the flow rate. As the Reynolds number and the dependency of the flow coefficient on the Reynolds number will be different for the fuel gas and the air, the air-fuel ratio tends not to stay constant at low demand.
3. The basic equations governing a mixing tee show that it cannot normally hold the air-fuel ratio constant if the temperature and composition of the air and fuel gas do not remain sufficiently constant. Weather is a major factor influencing the temperature and composition (humidity) of the air. The blower adds heat of compression to the air and can be a further reason for inconstancy of the air temperature.
A number of devices have been proposed to overcome the limitations of the conventional mixing tee. FIG. 2 shows one of these, a blender valve. Blender valves are disclosed in U.S. Pat. Nos. 1,980,770 and 2,243,704, for example. The two orifices and the pipe tee of FIG. 1 have been merged into a single device, the blender valve, construction shown in FIG. 2. The gas governor 10 is still present to insure equal pressure differences for the two gases being mixed together. The blender valve body 30 contains a rotatable sleeve 31 which cannot move up and down and a movable piston 32 which cannot rotate. The sleeve 31 and piston 32 each have three openings (a mixture opening, an air opening and a gas opening). The three openings are aligned to form two inlet ports for the two gases to be mixed and a single outlet port for the mixture. Rotating the sleeve 31 changes the relative area of the two inlet ports and consequently changes the ratio of the two gases in the mixture. As the piston 32 rises or falls in the cylinder all three ports vary in area, but the relative areas of the ports stay constant.
The piston 32 is automatically positioned vertically by a diaphragm 36. An impulse tube 34 connects one side of the diaphragm to the valve's air inlet. An opening 33 connects the other side of the diaphragm to the interior of the piston. The pressure difference across the diaphragm 36 drives the piston 32 up or down to maintain a constant pressure difference across the inlet ports. The pressure difference is set at a value large enough so that the effect of the gas governor error, discussed in problem 1 above, is negligible. However, the movable piston 32 does not solve problems 2 and 3 which were previously discussed herein. Problem 3 may be partially alleviated in the typical installation of a blender valve by the placement of the blower downstream from the blender valve so that the air temperature is not changed by the heat of compression. This is called a pull-through system. The conventional mixing tee uses a push-through system because the blower is upstream.
The blender valve of FIG. 2 is expensive to make because it requires a substantial amount of precision machining. The close fitting surfaces increase the need for maintenance because of fouling by dirty fuel, air, or corrosion. The lack of a perfect fit between the valve body and the sleeve and between the sleeve and the piston causes leakage between the air and fuel streams that will change the mixture composition at low demands. The result is that the initial and maintenance costs of a blender valve system will be higher than for a conventional mixing tee and the constancy of the mixture composition will not be as great as expected.
Another type of mixing device uses a characterized valve. Examples are described in U.S. Pat. Nos. 2,286,173 and 2,536,678. With these, as demand increases, a motor drives the air valve farther open in order to maintain a constant air pressure difference across the valve. The air valve, in turn, is mechanically linked to a characterized fuel gas valve. The characterized fuel valves have a complex mechanism that permit them to be adjusted to match the air valve so that the air-fuel ratio will stay constant as the demand changes. These overcome the mixing tee problems 1 and 2 previously discussed herein. However, it is difficult and time consuming to characterize them. The characterization is specific to the fuel and the air-fuel ratio. If either is changed, the valve has to be recharacterized. Again, this is expensive compared to a conventional mixing tee.